14.7.1.7 GNU Translate Tables

If you set the translate field of a pattern buffer to a translate
table, then the GNU Regex functions to which you’ve passed that
pattern buffer use it to apply a simple transformation
to all the regular expression and string characters at which they look.

A translate table is an array indexed by the characters in your
character set. Under the ASCII encoding, therefore, a translate
table has 256 elements. The array’s elements are also characters in
your character set. When the Regex functions see a character c,
they use translate[c] in its place, with one exception: the
character after a ‘\’ is not translated. (This ensures that, the
operators, e.g., ‘\B’ and ‘\b’, are always distinguishable.)

For example, a table that maps all lowercase letters to the
corresponding uppercase ones would cause the matcher to ignore
differences in case.5 Such a table would map all characters except lowercase letters
to themselves, and lowercase letters to the corresponding uppercase
ones. Under the ASCII encoding, here’s how you could initialize
such a table (we’ll call it case_fold):

You tell Regex to use a translate table on a given pattern buffer by
assigning that table’s address to the translate field of that
buffer. If you don’t want Regex to do any translation, put zero into
this field. You’ll get weird results if you change the table’s contents
anytime between compiling the pattern buffer, compiling its fastmap, and
matching or searching with the pattern buffer.